Add Mojo constant for "ash" and replace hardcoded strings

mash/common/BUILD.gn

Index: mash/common/BUILD.gn
diff --git a/mash/common/BUILD.gn b/mash/common/BUILD.gn
index 3f285f05d3f1a623179b2277e93294c2a4af3030..44d2bbc6aca436521e5c688f5d5dffa592f0fa27 100644
--- a/mash/common/BUILD.gn
+++ b/mash/common/BUILD.gn
@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/ui.gni")
+
 source_set("common") {
   sources = [
     "config.cc",
@@ -11,4 +13,8 @@ source_set("common") {
   deps = [
     "//base",
   ]
+
+  if (use_ash) {
+    deps += [ "//ash/public/interfaces" ]
+  }
 }